The present invention belongs to the field of shock protection devices for sensitive instruments. More specifically, it relates to a certain type of support device for a weighing cell and also to a balance with a stationary housing that incorporates the support device. In known support devices of the kind that the invention aims to improve, the weighing cell has one or more support posts by which the weighing cell is seated on a spring that is anchored at a stationary base or chassis portion, so that the one or more support posts of the weighing cell are movable in relation to the stationary chassis in a substantially vertical direction against the opposing force of the spring.
Measuring instruments that contain precision mechanisms can suffer damage if they are exposed to shocks in the course of being relocated or shipped, e.g., from being bumped, set down roughly, or even dropped. The risk of shock damage is particularly critical in high-resolution laboratory balances that incorporate levers, flexure pivots, parallelogram linkages and other delicate mechanical elements. Special shock protection measures have been attempted, so as to intercept excessive loads and thereby prevent damage to the sensitive parts of the apparatus. In the case of the aforementioned laboratory balances, the part that is most susceptible to damage is the weighing cell, including the force-introduction mechanism, particularly in regard to shocks in the downward vertical direction of the apparatus, i.e., in the same direction in which the device performs its actual weighing function. The most frequent problem with vertical shocks occurs when the apparatus is set down roughly after a change of place or after it has been lifted to clean the area underneath. In the field of balances, it is therefore a high-priority measure to keep downward-directed vertical shock loads from reaching the weighing cell. The problem is equally critical with shock forces acting in the opposite direction, for example if the balance is transported upside down or if it is turned on its head for cleaning.
One possibility of how shocks against the weighing cell can be softened is disclosed in CH-A-680877. This reference document describes a balance utilizing a printed circuit board, i.e., a component of the electronic circuitry contained in the balance, as a resilient support for the weighing cell. This design concept is conducive to a compact architecture of the instrument and provides a practical cushion against shock loads. However, in spite of the inherent damping properties of the printed circuit board, the system is susceptible to oscillations. For example, building vibrations can cause oscillations of the weighing cell. To correct this problem in high-resolution balances requires a time-consuming filtering process, and/or it is possible that the displayed weighing result will remain unstable.
The present invention therefore has the object of providing a shock-absorbing means for a weighing cell that is effective against downward- as well as upward-directed vertical shocks and overcomes the aforementioned drawbacks while still allowing a compact, low-profile configuration of the apparatus.
In accordance with the present invention, the foregoing objective can be realized in a support device which, as described at the beginning, has one or more support posts that are connected to and support the weighing cell, where each support post rests on a support spring that is anchored at a stationary chassis portion, with the support post being movable in relation to the stationary chassis in a substantially vertical direction against the opposing force of the support spring. In addition, the support device of the present invention has the distinguishing features that each of the support springs on which the one or more support posts are resting is biased with an upward pre-tensioning force against a surface portion of the stationary base or chassis, and that the support post can lift off from its resting place on the support spring, but is constrained to a limited range of upward vertical movement by a guide member that is attached to the stationary chassis. Also included within the scope of the invention is a balance with a stationary housing that surrounds the weighing cell, where a bottom or floor portion of the housing is designed to accommodate the inventive support device and, more specifically, where the bottom or floor of the housing constitutes the stationary chassis on which the spring is anchored.
The inventive concept where the weighing cell is supported by one or more support posts resting on a support spring that is upward-biased with a pre-tensioning force provides a quasi-rigid support base for the weighing cell as long as the downward force transmitted by the support post(s) does not exceed the pre-tensioning force of the support spring. When the pre-tensioning force is exceeded, i.e., when a downward-directed shock is resiliently absorbed, the weighing cell deflects approximately in a mode of parallel motion, because of the guide member(s) constraining the support post(s). In other words, the weighing cell will not tip into a slanted position and, therefore, very little lateral clearance is required, which is again conducive to a compact design. As a further advantage, after the peak forces have subsided, the weighing cell will return to its exact previous position. In addition, the weighing cell can also lift off from its rest position on the support spring, so that an upward-directed vertical shock can likewise be absorbed.
In an advantageous development of the invention, the guide members are designed as guide springs with the capability of resiliently absorbing upward-directed vertical shocks, without the need for additional parts nor an increase in the profile height of the apparatus.
A particularly simple and at the same time space-saving configuration is achieved by designing the one or more support springs as leaf springs. Each of the support posts that support the weighing cell can be seated on a leaf spring in a straightforward manner, in particular with a design where the support post has two sections of different thickness, whereby a shoulder is formed by which the post rests on the support spring, with the thinner section of the bolt protruding through the support spring and the bottom end of the post being fastened to the guide member. In a preferred embodiment of the invention, each support spring is arranged to work together with a guide spring as a pair, in which the two springs run parallel to each other, with the advantageous result of a very low profile height.
By designing the guide members in the form of leaf springs, it is possible to combine multiple guide members that are used together in one support device into an integral die-punched unit. With appropriately shaped bends in the leaf springs, and with a suitable choice of the leaf dimensions, thickness and width in particular, it is possible to match the spring tension to given design constraints, primarily to the mass of the weighing cell and associated parts, and to performance requirements, in particular the magnitude of the shocks that the springs must be capable of absorbing. The design requirements for the support springs are analogous to the guide springs.
In a balance, the stationary base or chassis to which the support device is attached, is constituted by the stationary balance housing that encloses the weighing cell. In balances with a high measurement resolution, the space inside the housing is often subdivided into compartments in order to shield the weighing cell from being influenced by factors originating from other parts of the balance. Consequently, it is advantageous, for example, to use a design where the volume of the space surrounding the cell is minimized. In a preferred embodiment of a balance with the inventive support device, a minimum-volume configuration is achieved by a split-level design of the floor of the stationary housing, i.e., different parts of the floor being located at different levels. The purpose is to create a hollow space under a raised floor area below the weighing cell. This hollow space, which can be closed by a cover plate, can accommodate the support springs and the guide members in addition to other components of the balance. The raised floor area has openings through which the posts of the support device protrude into the hollow space. The support springs as well as the guide members are preferably of a leaf-spring design and are accommodated inside the hollow space, attached to the stationary housing floor from below.
In another embodiment, the stationary floor of the balance housing runs in a level plane, where the support springs and guide members are arranged above the floor. In this case it is advantageous to provide the floor with closable openings in the areas where the guide members are attached to the support posts in order to provide access for assembly and disassembly.
The invention is described below with reference to the drawing figures that illustrate preferred embodiments where the invention is incorporated in a micro-analytical balance and a top-loading precision balance. It must be understood, however, that the invention is not restricted to the specific design configurations described and illustrated herein, but that variations and adaptations that present themselves from the disclosure in its entirety are included within the scope of the invention.